S02 THE PALEONTOLOGY OF MINNESOTA. 



[Oncoceras pandlon. 



more rapid on the ventral than the dorsal surface and hence the curvature is 

 greater on the outer than on the inner margin of the shell. Dorsal and lateral 

 surfaces very broad; ventral surface narrow and somewhat compressed laterally. 

 The transverse section of the shell is therefore very broadly ovate, almost circular, 

 the dorso-ventral and lateral diameters being to each other as 13 to 12 at the last 

 septum; as 12 to 11 at the first septum exposed (the eighth from the aperture). 

 The septa are regularly and not deeply concave ; the sutures regularly transverse 

 and simple, without lobes or saddles and the air-chambers comparatively broad and 

 of subequal depth on ventral and dorsal margins alike. The average depth of these 

 chambers is 3 to 4 mm., eight of them occuping a length of 32 mm. Surface 

 smooth, covered with obscure concentric lines which follow the outline of the 

 apertural margin and are, hence, bent backward over the venter. The lateral 

 length of the specimen described, from the aperture to the eighth or terminal air- 

 chambei 1 , is 59 mm., its dorsal length 51 mm. and the ventral length 57 mm. The 

 lateral length of the body-chamber is 20 mm. The transverse section at the eighth 

 septum measures 12 by 11 mm.; at the base of the body-chamber 26x24 mm.; at the 

 bottom of the subapertural constriction 28x34 mm. This species is charaterized by 

 its very broad dorsum, rapid expansion over the later air-'chambers, the regularity 

 and considerable depth of the latter. It is most nearly allied to Oncoceras constric- 

 tum Hall, of the Trenton limestone of Middleville, N. Y., but is less arcuate than 

 that species, and the expansion of the tube is less abrupt and less ventricose on the 



dorsal surface. 







Formation and locality. In the Galena limestone at Hacler, Goodhue county, Minnesota. 

 Museum Register, No. 243. 



ONCOCERAS PANDION Hall, 1861. 



PLATE LVIII, FIGS. 4-6. 



Oncocerat pandion HALL, 1861. Eept. Supt. Geol. Surv. Wisconsin, p. 45. 



Original Description: "Shell robust, strongly curved, very rapidly expanding 

 to near the outer chamber which gently decreases in size for nearly two-thirds of 

 its length and then becomes suddenly constricted to nearly one-half its former 

 dimensions ; broadly ovate or subcircular, the [greatest] diameter in the dorso- 

 ventral direction. Septa moderately distant, strongly curved forwards on the 

 dorsal side, the greatest concavity on the ventral side. Siphuncle large, dorsal. 

 Surface unknown." 



To this species I refer a few specimens characterized by the great inequality in 

 the curves of the ventral and dorsal surfaces, the latter being very gentle, while 



